The heat development, which needs only heating for the formation of an image, is already known. There are known heat-developing processes for black-and-white image formation and color image formation, and also well-known transfer-type heat-developable light-sensitive materials in which the heat-developed image is transferred onto an image-receiving layer.
Conventionally known as the support for the heat-developable light-sensitive material include various materials such as paper, synthetic paper, laminated paper, plastic film, and the like, which are selectively used according to purposes. The particularly preferred among these support materials is polyethylene terephthalate film (PET). This is commonly used from the economical and ease-of-handling point of view. The use of PET as the support for the heat-developable light-sensitive material, however, is disadvantageous because PET film softens as its temperature rises, and tends to thermally shrink at a heat-developing temperature (normally 80.degree. to 200.degree. C.), so that it sometimes causes jamming or creasing trouble.
In contrast, paper is advantageous in that it neither softens nor shrinks at a high temperature, and for cost reduction as well.
However, where paper is used as the support for the heat-developable light-sensitive material, it is liable to curl under low or high humidity conditions; the phenomenon becomes conspicuous particularly at the time of heat development.
The curl, in the diffusion transfer of a dye image from the light-sensitive material onto an image-receiving material, causes the transferred image to have uneven densities as well as jamming trouble to occur inside the processing device. In order to prevent such troubles attributable to the curl at the time of the heat-development, JP O.P.I. No. 205935/1986 discloses a technique to provide a hydrophilic colloid layer on the reverse side of the light-sensitive material. However, the hydrophilic colloid on the reverse side, because of its hygroscopicity, may cause the heat-developable light-sensitive material to allow transfer of its effective constituent to the reverse side during its storage or to have sticking trouble in a high humidity condition, and is by no means sufficient for improving the light-sensitive material's curl characteristic in heat development while keeping its curl characteristic stable against changes in the humidity at room temperature.
Where a paper support is used for the heat-developable light-sensitive material, if a light-sensitive layer is coated directly on the support, some of the constituents of the light-sensitive layer permeate into the support to thereby cause the formation of a mottled image. In order to prevent the permeation of light-sensitive constituents into the paper support, JP O.P.I. No. 38934/1988 discloses a technique to provide a hydrophobic polymer layer on the paper support. This technique surely solves the above permeation problem, but the use of the technique alone is not sufficient for improving the aforementioned curl problem in heat development.